Boundary acoustic wave device

ABSTRACT

A dielectric substance is laminated on one surface of a piezoelectric substance, and an IDT and reflectors are disposed as electrodes at a boundary between the piezoelectric substance and the dielectric substance, and the thickness of the electrodes is determined so that the acoustic velocity of the Stoneley wave is decreased less than that of a slow transverse wave propagating through the dielectric substance and that of a slow transverse wave propagating through the piezoelectric substance, thereby forming a boundary acoustic wave device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boundary acoustic wave device using aStoneley wave, and more particularly, to a boundary acoustic wave deviceusing a Stoneley wave and including electrodes that are disposed at aboundary between a piezoelectric substance and a dielectric substance.

2. Description of the Related Art

Heretofore, various surface acoustic wave devices have been used for RFand IF filters in mobile phones, resonators in VCOs, VIF filters intelevisions, and other devices. Surface acoustic wave devices use asurface acoustic wave, such as a Rayleigh wave or a first leaky wave,which propagates along a surface of a medium.

Since the acoustic wave propagating along a surface of a medium, asurface acoustic wave is sensitive to changes in the surface conditionof the medium. Accordingly, in order to protect a surface of a mediumalong which a surface acoustic wave propagates, a surface acoustic waveelement is often hermetically sealed in a package in which a cavityportion is provided so as to face the wave-propagating surface. Sincethe package having a cavity portion as described above is used, the costof the surface acoustic wave device is increased. In addition, since thesize of the package is much larger than that of the surface acousticwave element, the size of the surface acoustic wave device is increased.

On the other hand, among acoustic waves, in addition to theabove-described surface acoustic waves, a boundary acoustic wavepropagates along a boundary between solid substances.

For example, in “Piezoelectric Acoustic Boundary Waves Propagating Alongthe Interface Between SiO₂ and LiTaO₃” IEEE Trans. Sonics andUltrasonics, VOL. SU-25, No. 6, 1978 IEEE (non-patent document 1), aboundary acoustic wave device is disclosed in which an IDT is disposedon a 126° rotated Y plate X-propagation LiTaO₃ substrate and a SiO₂ filmhaving a predetermined thickness is disposed on the LiTaO₃ substrate andthe IDT. In non-patent document 1, an SV+P type boundary acoustic wave,which is a so-called Stoneley wave, is propagated. In non-patentdocument 1, when the thickness of the above SiO₂ film is set to 1.0λ (λindicates the wavelength of a boundary acoustic wave), anelectromechanical coefficient of 2% is obtained.

The boundary acoustic wave propagates in the state in which energy isconcentrated on a boundary portion between solid substrates. Hence,since energy is not substantially present on the bottom surface of theabove LiTaO₃ substrate and the surface of the SiO₂ film, the propertiesare not changed by changes in surface conditions of the substrate andthe thin film.

Accordingly, a package having a cavity portion is not required, andhence the size of the boundary acoustic wave device is reduced.

In addition, in “Piezoelectric Boundary Wave in a Substrate with aLayered Structure” authored by Chujo, Yamanouchi, and Shibayama,Research Institute of Electrical Communication, US80-4, 1980 (non-patentdocument 2), a boundary acoustic wave called a Stoneley wave isdisclosed which propagates in the structure in which a SiO₂ film isdisposed on a 128° rotated Y plate X-propagation LiNbO₃ substrate.According to the analysis of the non-patent document 2, it is shown thatwhen the SiO₂ is in its natural state, since the displacement is notconcentrated on the boundary between the SiO₂ layer and the LiNbO₃substrate, a boundary acoustic wave is not generated, and that when theLame constant indicating the elasticity of SiO₂ is changed from aninherent value of 0.3119×10¹¹ N/m² to 0.4679×10¹¹ N/m², the displacementis concentrated on the boundary, such that a boundary acoustic wave isgenerated. In addition, according to the experimental result of thenon-patent document 2, it has also been disclosed that even whenconditions for forming the SiO₂ layer are changed, a SiO₂ film cannot beformed in which a boundary acoustic wave propagates.

In a boundary acoustic wave device, a large electromechanicalcoefficient, a small propagation loss, a small power flow angle, and asmall temperature coefficient of frequency have been required. The losscaused by the propagation of a boundary acoustic wave, that is, thepropagation loss, may degrade the insertion loss of a boundary acousticwave filter or may also degrade the resonant resistance or the impedanceratio of a boundary acoustic wave resonator, the impedance ratio being aratio between the impedance at a resonant frequency and that at anantiresonant frequency. Hence, the propagation loss is preferablydecreased to as small as possible.

The power flow angle is an angle indicating the difference between thedirection of the phase velocity of a boundary acoustic wave and thedirection of the group velocity of energy thereof. When the power flowangle is large, it is necessary to obliquely dispose an IDT inconformity with the power flow angle. Hence, the design of theelectrodes is complicated. In addition, a loss caused by a deviation inthe angle is likely to be generated.

Furthermore, when an operating frequency of a boundary acoustic wavedevice is changed by the temperature, practical pass band and stop bandare decreased in a boundary acoustic wave filter. With a resonator, whenan oscillation circuit is formed, the above-described change inoperating frequency caused by the temperature results in abnormaloscillation. Hence, the change in frequency per degree centigrade, whichis TCF, is preferably decreased to as small as possible.

For example, when reflectors are disposed along a propagation directionand outside a region in which a transmitting IDT and a receiving IDT areprovided, which transmits and receives a boundary acoustic wave,respectively, a low-loss resonator type filter can be formed. The bandwidth of this resonator type filter depends on the electromechanicalcoefficient of a boundary acoustic wave. When the electromechanicalcoefficient k² is large, a broadband filter is obtained, and when theelectromechanical coefficient k² is small, a narrowband filter isobtained. Thus, the electromechanical coefficient k² of a boundaryacoustic wave used for a boundary acoustic wave device must beappropriately determined in accordance with its application. When an RFfilter for mobile phones is formed, the electromechanical coefficient k²is required to be at least 5%.

However, in the boundary acoustic wave device using a Stoneley wave,which is disclosed in non-patent document 1, the electromechanicalcoefficient k² is small, such as 2%.

In addition, in the SiO₂/LiNbO₃ structure disclosed in the non-patentdocument 2, a LiNbO₃ substrate having large piezoelectric properties isused. Hence, compared to the boundary acoustic wave device described inthe non-patent document 1, a larger electromechanical coefficient k² canbe obtained. However, it is difficult to form a SiO₂ film such that aboundary acoustic wave propagates, and the non-patent document 2discloses no measurement results of a Stoneley wave after actuallyforming the SiO₂ film.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a boundary acoustic wave device using aStoneley wave which has a sufficiently large electromechanicalcoefficient, small propagation loss, small power flow angle, and a smalltemperature coefficient of frequency, and which can be manufactured by asimple method.

In accordance with a first preferred embodiment of the presentinvention, a boundary acoustic wave device using a Stoneley waveincludes a piezoelectric substance, a dielectric substance laminated onone surface of the piezoelectric substance, and electrodes provided at aboundary between the piezoelectric substance and the dielectricsubstance. In the boundary acoustic wave device according to thispreferred embodiment, the thickness of the electrodes is determined suchthat the acoustic velocity of the Stoneley wave is less than that of aslow transverse wave propagating through the dielectric substance andthat of a slow transverse wave propagating through the piezoelectricsubstance.

In accordance with a second preferred embodiment of the presentinvention, a boundary acoustic wave device using a Stoneley waveincludes a piezoelectric substance, a dielectric substance laminated onone surface of the piezoelectric substance, and electrodes provided at aboundary between the piezoelectric substance and the dielectricsubstance. In the boundary acoustic wave device according to thispreferred embodiment, the duty ratio of strips defining the electrodesis determined such that the acoustic velocity of the Stoneley wave isless than that of a slow transverse wave propagating through thedielectric substance and that of a slow transverse wave propagatingthrough the piezoelectric substance.

In accordance with a third preferred embodiment of the presentinvention, a boundary acoustic wave device using a Stoneley waveincludes a piezoelectric substance primarily including LiNbO₃, adielectric substance laminated on one surface of the piezoelectricsubstance, and electrodes provided at a boundary between thepiezoelectric substance and the dielectric substance. In the boundaryacoustic wave device according to this preferred embodiment, Eulerangles (φ, θ, ψ) of the piezoelectric substance primarily includingLiNbO₃ are in the respective ranges shown in the following Table 1, anda Stoneley wave having an acoustic velocity of 3,757 m/sec or less isused.

TABLE 1 φ (°) θ (°) ψ (°) 30 90 225 30 270 135 30 270 315 90 90 135 9090 315 90 270 45 90 270 225 150 90 45 150 90 225 150 270 135 150 270 315210 90 135 210 90 315 210 270 45 210 270 225 270 90 45 270 90 225 270270 135 270 270 315 330 90 135 330 90 315 330 270 45 330 270 225

In the second and the third preferred embodiments of the presentinvention, the thickness of the electrodes is preferably determined suchthat the acoustic velocity of the Stoneley wave is less than that of theslow transverse wave propagating through the dielectric substance andthat of the slow transverse wave propagating through the piezoelectricsubstance.

In the boundary acoustic wave device according to the third preferredembodiment of the present Invention, the duty ratio of strips formingthe electrodes is preferably determined such that the acoustic velocityof the Stoneley wave is less than that of a slow transverse wavepropagating through the dielectric substance and that of a slowtransverse wave propagating through the piezoelectric substance.

In accordance with a fourth preferred embodiment of the presentinvention, a boundary acoustic wave device using a Stoneley waveincludes a piezoelectric substance primarily including LiNbO₃, adielectric substance laminated on one surface of the piezoelectricsubstance, and electrodes provided at a boundary between thepiezoelectric substance and the dielectric substance. In the boundaryacoustic wave device described above, when the density of theelectrodes, the thickness thereof, and the wavelength of the Stoneleywave are represented by ρ (kg/m³), H (λ) and λ, respectively,H>1/[1/(3×10⁷×ρ^(−2.22)+0.017)−0.4] is maintained.

In the fourth preferred embodiment of the present invention, the densityρ of the electrodes is preferably set to 4,711 kg/m³ or more.

In accordance with a fifth preferred embodiment of the presentinvention, a boundary acoustic wave device using a Stoneley waveincludes a piezoelectric substance primarily including LiNbO₃, adielectric substance laminated on one surface of the piezoelectricsubstance, and electrodes provided at a boundary between thepiezoelectric substance and the dielectric substance. In the boundaryacoustic wave device described above, when the density of theelectrodes, the thickness thereof, and the wavelength of the Stoneleywave are represented by ρ (kg/m³), H (λ), and λ, respectively, H>0.03λand ρ>2,699 kg/m³ is maintained.

In the boundary acoustic wave device according to one of the first tothe fifth preferred embodiments of the present invention, preferably theelectrodes primarily include an electrode layer including at least onematerial selected from the group consisting of Ag, Au, Cu, Fe, Mo, Ni,Ta, W, Ti, and Pt.

The boundary acoustic wave device according to the first preferredembodiment of the present invention includes a piezoelectric substance,a dielectric substance laminated on one surface of the piezoelectricsubstance, and electrodes disposed at a boundary between thepiezoelectric substance and the dielectric substance, and in theabove-described boundary acoustic wave device, the thickness of theelectrodes is preferably determined such that the acoustic velocity of aStoneley wave is less than that of a slow transverse wave propagatingthrough the dielectric substance and that of a slow transverse wavepropagating through the piezoelectric substance.

In addition, the boundary acoustic wave device according to the secondpreferred embodiment of the present invention includes a piezoelectricsubstance, a dielectric substance laminated on one surface of thepiezoelectric substance, and electrodes disposed at a boundary betweenthe piezoelectric substance and the dielectric substance, and in theabove-described boundary acoustic wave device, the duty ratio of stripsforming the electrodes is determined such that the acoustic velocity ofa Stoneley wave is less than that of a slow transverse wave propagatingthrough the dielectric substance and that of a slow transverse wavepropagating through the piezoelectric substance.

Hence, according to the first or second preferred of the presentinvention, since the thickness of the electrodes or the duty ratio ofthe strips thereof is determined as described above, a boundary acousticwave device is provided in which the Stoneley wave propagates throughthe dielectric substance and the piezoelectric substance.

The boundary acoustic wave device according to the third preferred ofthe present invention includes a piezoelectric substance primarilyincluding LiNbO₃, a dielectric substance laminated on one surface of thepiezoelectric substance, and electrodes disposed at a boundary betweenthe piezoelectric substance and the dielectric substance, and in theabove-described boundary acoustic wave device, Euler angles (φ, θ, ψ) ofthe piezoelectric substance are in the respective ranges shown in Table1, and a Stoneley wave having an acoustic velocity of about 3,757 m/secor less is preferably used. Accordingly, as apparent from examples to bedescribed later, spurious responses are effectively suppressed, and theelectromechanical coefficient k² of the Stoneley wave is increased.

In the boundary acoustic wave device according to the second or thethird preferred embodiment of the present invention, when the thicknessof the electrodes or the duty ratio is determined such that the acousticvelocity of the Stoneley wave is less than that of the slow transversewave propagating through the dielectric substance and that of the slowtransverse wave propagating through the piezoelectric substance, aboundary acoustic wave device is provided in which the Stoneley wavereliably propagates along the boundary between the dielectric substanceand the piezoelectric substance.

The boundary acoustic wave device according to the fourth preferredembodiment of the present invention includes a piezoelectric substanceprimarily including LiNbO₃, a dielectric substance laminated on onesurface of the piezoelectric substance, and electrodes provided at aboundary between the piezoelectric substance and the dielectricsubstance, and when the density of the electrodes, the thicknessthereof, and the wavelength of the Stoneley wave are represented by ρ(kg/m³), H (λ), and λ, respectively, H>1/[1/(3×10⁷×ρ^(−2.22)+0.017)−0.4]is maintained. Thus, a boundary acoustic wave device is provided whichuses a Stoneley wave having an appropriate electromechanical coefficientk². In particular, when the density ρ of the electrodes is 4,711 kg/m³or more, the thickness of the electrodes at a propagation loss of 0 isdecreased, and thus, the electrodes are easily formed.

The boundary acoustic wave device according to the fifth preferredembodiment of the present invention includes a piezoelectric substanceprimarily including LiNbO₃, a dielectric substance laminated on onesurface of the piezoelectric substance, and electrodes provided at aboundary between the piezoelectric substance and the dielectricsubstance, and when the density of the electrodes, the thicknessthereof, and the wavelength of the Stoneley wave are represented by ρ(kg/m³), H (λ), and λ, respectively, H>0.03λ and ρ>2,699 kg/m³ ismaintained. Thus, a boundary acoustic wave device is provided which useselectrodes made of a material that is heavier than Al and in which theStoneley wave propagates.

In various preferred embodiments of the present invention, when theelectrodes are each primarily made of an electrode layer including atleast one material selected from the group consisting of Ag, Au, Cu, Fe,Mo, Ni, Ta, W, Ti, and Pt, in accordance with the present invention, aboundary acoustic wave device using a Stoneley wave is provided. Inaddition, when at least one second electrode layer including a metalother than that forming the above-described electrode layer is furtherprovided, by selecting a metal material forming the second electrodelayer, the adhesion of the electrode with the dielectric substance orthe piezoelectric substance is increased, or the electric powerresistance is enhanced.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front cross-sectional view showing a boundary acoustic wavedevice according to a preferred embodiment of the present invention.

FIG. 2 is a schematic plan view showing an IDT and reflectors, which aredefined by electrodes, of a boundary acoustic wave device of a preferredembodiment according to the present invention.

FIG. 3 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A1 shown in Table 1 formed in Example 1.

FIG. 4 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A2 shown in Table 1 formed in Example 1.

FIG. 5 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A3 shown in Table 1 formed in Example 1.

FIG. 6 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A4 shown in Table 1 formed in Example 1.

FIG. 7 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A5 shown in Table 1 formed in Example 1.

FIG. 8 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A6 shown in Table 1 formed in Example 1.

FIG. 9 is a graph showing impedance-frequency characteristics of aboundary acoustic wave device A7 shown in Table 1 formed in Example 1.

FIG. 10 is a graph showing the relationship between an Euler angle φ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 11 is a graph showing the relationship between an Euler angle φ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (φ, 0°, 0°), and a SiO₂film is then formed thereon.

FIG. 12 is a graph showing the relationship between an Euler angle φ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 13 is a graph showing the relationship between an Euler angle φ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (φ, 0°, 0°), and aSiO₂ film is then formed thereon.

FIG. 14 is a graph showing the relationship between an Euler angle φ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 15 is a graph showing the relationship between an Euler angle φ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 16 is a graph showing the relationship between an Euler angle φ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (φ, 0°, 90°), and a SiO₂film is then formed thereon.

FIG. 17 is a graph showing the relationship between an Euler angle φ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 18 is a graph showing the relationship between an Euler angle φ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (φ, 0°, 90°), and aSiO₂ film is then formed thereon.

FIG. 19 is a graph showing the relationship between an Euler angle φ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 0°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 20 is a graph showing the relationship between an Euler angle φ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 21 is a graph showing the relationship between an Euler angle φ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (φ, 90°, 0°), and a SiO₂film is then formed thereon.

FIG. 22 is a graph showing the relationship between an Euler angle φ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 23 is a graph showing the relationship between an Euler angle φ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (φ, 90°, 0°), and aSiO₂ film is then formed thereon.

FIG. 24 is a graph showing the relationship between an Euler angle φ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 0°), and a SiO₂ film is thenformed thereon.

FIG. 25 is a graph showing the relationship between an Euler angle φ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 26 is a graph showing the relationship between an Euler angle φ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (φ, 90°, 90°), and a SiO₂film is then formed thereon.

FIG. 27 is a graph showing the relationship between an Euler angle φ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 28 is a graph showing the relationship between an Euler angle φ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (φ, 90°, 90°), and aSiO₂ film is then formed thereon.

FIG. 29 is a graph showing the relationship between an Euler angle φ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (φ, 90°, 90°), and a SiO₂ film is thenformed thereon.

FIG. 30 is a graph showing the relationship between an Euler angle θ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 31 is a graph showing the relationship between an Euler angle θ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (0°, θ, 0°), and a SiO₂film is then formed thereon.

FIG. 32 is a graph showing the relationship between an Euler angle θ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 33 is a graph showing the relationship between an Euler angle θ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (0°, θ, 0°), and aSiO₂ film is then formed thereon.

FIG. 34 is a graph showing the relationship between an Euler angle θ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 35 is a graph showing the relationship between an Euler angle θ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 36 is a graph showing the relationship between an Euler angle θ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (0°, θ, 90°), and a SiO₂film is then formed thereon.

FIG. 37 is a graph showing the relationship between an Euler angle θ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 38 is a graph showing the relationship between an Euler angle θ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (0°, θ, 90°), and aSiO₂ film is then formed thereon.

FIG. 39 is a graph showing the relationship between an Euler angle θ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 40 is a graph showing the relationship between an Euler angle θ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 41 is a graph showing the relationship between an Euler angle θ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (90°, θ, 0°), and a SiO₂film is then formed thereon.

FIG. 42 is a graph showing the relationship between an Euler angle θ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 43 is a graph showing the relationship between an Euler angle θ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (90°, θ, 0°), and aSiO₂ film is then formed thereon.

FIG. 44 is a graph showing the relationship between an Euler angle θ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 0°), and a SiO₂ film is thenformed thereon.

FIG. 45 is a graph showing the relationship between an Euler angle θ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 46 is a graph showing the relationship between an Euler angle θ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (90°, θ, 90°), and a SiO₂film is then formed thereon.

FIG. 47 is a graph showing the relationship between an Euler angle θ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 48 is a graph showing the relationship between an Euler angle θ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (90°, θ, 90°), and aSiO₂ film is then formed thereon.

FIG. 49 is a graph showing the relationship between an Euler angle θ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, θ, 90°), and a SiO₂ film is thenformed thereon.

FIG. 50 is a graph showing the relationship between an Euler angle ψ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 51 is a graph showing the relationship between an Euler angle ψ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (0°, 0°, ψ), and a SiO₂film is then formed thereon.

FIG. 52 is a graph showing the relationship between an Euler angle ψ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 53 is a graph showing the relationship between an Euler angle ψ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (0°, 0°, ψ), and aSiO₂ film is then formed thereon.

FIG. 54 is a graph showing the relationship between an Euler angle ψ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 55 is a graph showing the relationship between an Euler angle ψ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 56 is a graph showing the relationship between an Euler angle ψ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (0°, 90°, ψ), and a SiO₂film is then formed thereon.

FIG. 57 is a graph showing the relationship between an Euler angle ψ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 58 is a graph showing the relationship between an Euler angle ψ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (0°, 90°, ψ), and aSiO₂ film is then formed thereon.

FIG. 59 is a graph showing the relationship between an Euler angle ψ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (0°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 60 is a graph showing the relationship between an Euler angle ψ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 61 is a graph showing the relationship between an Euler angle ψ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (90°, 0°, ψ), and a SiO₂film is then formed thereon.

FIG. 62 is a graph showing the relationship between an Euler angle ψ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 63 is a graph showing the relationship between an Euler angle ψ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (90°, 0°, ψ), and aSiO₂ film is then formed thereon.

FIG. 64 is a graph showing the relationship between an Euler angle ψ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 0°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 65 is a graph showing the relationship between an Euler angle ψ andthe acoustic velocity V in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 66 is a graph showing the relationship between an Euler angle ψ andthe electromechanical coefficient k² in the structure in which Auelectrodes are formed on a LiNbO₃ substrate of (90°, 90°, ψ), and a SiO₂film is then formed thereon.

FIG. 67 is a graph showing the relationship between an Euler angle ψ andthe propagation loss α in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 68 is a graph showing the relationship between an Euler angle ψ andthe temperature coefficient of frequency TCF in the structure in whichAu electrodes are formed on a LiNbO₃ substrate of (90°, 90°, ψ), and aSiO₂ film is then formed thereon.

FIG. 69 is a graph showing the relationship between an Euler angle ψ andthe power flow angle PFA in the structure in which Au electrodes areformed on a LiNbO₃ substrate of (90°, 90°, ψ), and a SiO₂ film is thenformed thereon.

FIG. 70 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ag.

FIG. 71 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ag.

FIG. 72 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ag.

FIG. 73 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ag.

FIG. 74 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Al.

FIG. 75 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Al.

FIG. 76 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Al.

FIG. 77 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Al.

FIG. 78 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Au.

FIG. 79 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Au.

FIG. 80 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Au.

FIG. 81 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Au.

FIG. 82 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Cr.

FIG. 83 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Cr.

FIG. 84 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Cr.

FIG. 85 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Cr.

FIG. 86 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Cu.

FIG. 87 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Cu.

FIG. 88 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Cu.

FIG. 89 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Cu.

FIG. 90 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Fe.

FIG. 91 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Fe.

FIG. 92 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Fe.

FIG. 93 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Fe.

FIG. 94 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Mo.

FIG. 95 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Mo.

FIG. 96 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Mo.

FIG. 97 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Mo.

FIG. 98 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ni.

FIG. 99 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ni.

FIG. 100 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ni.

FIG. 101 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ta.

FIG. 102 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ta.

FIG. 103 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ta.

FIG. 104 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ta.

FIG. 105 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from W.

FIG. 106 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from W.

FIG. 107 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from W.

FIG. 108 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from W.

FIG. 109 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ti.

FIG. 110 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Ti.

FIG. 111 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ti.

FIG. 112 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Ti.

FIG. 113 is a graph showing the relationship between the electrodethickness and the acoustic velocity of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Pt.

FIG. 114 is a graph showing the relationship between the electrodethickness and the propagation loss α of a Stoneley wave in a boundaryacoustic wave device formed in Example 3 in which the electrodes areformed from Pt.

FIG. 115 is a graph showing the relationship between the electrodethickness and the electromechanical coefficient k² of a Stoneley wave ina boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Pt.

FIG. 116 is a graph showing the relationship between the electrodethickness and the temperature coefficient of frequency TCF of a Stoneleywave in a boundary acoustic wave device formed in Example 3 in which theelectrodes are formed from Pt.

FIG. 117 is a graph showing the relationship between the density ofelectrodes and the thickness thereof at which the propagation loss α ofa Stoneley wave is 0, the relationship being obtained when boundaryacoustic wave devices are formed by variously changing the density ofthe electrodes in Example 3.

FIG. 118 is a graph showing the relationship between the acousticvelocity and an Euler angle φ in a boundary acoustic wave device formedin Example 4 in which Au electrodes and a SiO₂ film are formed on aLiNbO₃ substrate of Euler angles (φ, 30°, 0°).

FIG. 119 is a graph showing the relationship between the temperaturecoefficient of frequency TCF and an Euler angle φ in a boundary acousticwave device formed in Example 4 in which Au electrodes and a SiO₂ filmare formed on a LiNbO₃ substrate of Euler angles (φ, 30°, 0°).

FIG. 120 is a graph showing the relationship between theelectromechanical coefficient k² and an Euler angle φ in a boundaryacoustic wave device formed in Example 4 in which Au electrodes and aSiO₂ film are formed on a LiNbO₃ substrate of Euler angles (φ, 30°, 0°).

FIG. 121 is a graph showing the relationship between the power flowangle PFA and an Euler angle φ in a boundary acoustic wave device formedin Example 4 in which Au electrodes and a SiO₂ film are formed on aLiNbO₃ substrate of Euler angles (φ, 30°, 0°).

FIG. 122 is a graph showing the relationship between the propagationloss α and an Euler angle φ in a boundary acoustic wave device formed inExample 4 in which Au electrodes and a SiO₂ film are formed on a LiNbO₃substrate of Euler angles (φ, 30°, 0°).

FIG. 123 is a graph showing the relationship between the acousticvelocity and an Euler angle ψ in a boundary acoustic wave device formedin Example 4 in which Au electrodes and a SiO₂ film are formed on aLiNbO₃ substrate of Euler angles (0°, 30°, ψ).

FIG. 124 is a graph showing the relationship between the temperaturecoefficient of frequency TCF and an Euler angle v in a boundary acousticwave device formed in Example 4 in which Au electrodes and a SiO₂ filmare formed on a LiNbO₃ substrate of Euler angles (0°, 30°, ψ).

FIG. 125 is a graph showing the relationship between theelectromechanical coefficient k² and an Euler angle ψ in a boundaryacoustic wave device formed in Example 4 in which Au electrodes and aSiO₂ film are formed on a LiNbO₃ substrate of Euler angles (0°, 30°, ψ).

FIG. 126 is a graph showing the relationship between the power flowangle PFA and an Euler angle ψ in a boundary acoustic wave device formedin Example 4 in which Au electrodes and a SiO₂ film are formed on aLiNbO₃ substrate of Euler angles (0°, 30°, ψ).

FIG. 127 is a graph showing the relationship between the propagationloss α and an Euler angle ψ in a boundary acoustic wave device formed inExample 4 in which Au electrodes and a SiO₂ film are formed on a LiNbO₃substrate of Euler angles (0°, 30°, ψ).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, with reference to the figures, particular examples ofpreferred embodiments of the present invention will be described so asto facilitate understanding of the present invention.

In order to enable a boundary acoustic wave to propagate between twosolid layers, energy of the boundary acoustic wave must be concentratedbetween the solid layers.

In general, when a high acoustic velocity region and a low acousticvelocity region are present, a wave is concentrated on the low acousticvelocity region and propagates therein. Accordingly, the inventor of thepresent invention discovered that the condition in which energy isconcentrated between two solid layers can be satisfied when the acousticvelocity of a boundary acoustic wave propagating between the solidlayers is decreased by increasing the thickness of electrodes made of ametal material, such as Au or Cu, which has a high density and a lowacoustic velocity as an electrode material provided between the twosolid layers, and as a result, the present invention was developed.

Heretofore, it has been known that three types of bulk waves, that is, alongitudinal wave, a fast transverse wave, and a slow transverse waveare present, and that they are called a P wave, an SH wave, and an SVwave, respectively. Whether the SH wave or the SV wave becomes a slowtransverse wave is determined by the anisotropic properties of a basematerial. Among the above three types of bulk waves, a bulk wave havingthe lowest acoustic velocity is a slow transverse wave. When the solidsubstance is an isotropic substance such as SiO₂, since only one type oftransverse wave propagates therethrough, this transverse wave is a slowtransverse wave.

On the other hand, in a boundary acoustic wave propagating through ananisotropic base material, such as a piezoelectric substrate, in mostcases, three displacement components of the P wave, SH wave, and SV wavepropagate while being coupled with each other, and the type of boundaryacoustic wave is determined by the primary component. For example, theabove Stoneley wave is a boundary acoustic wave primarily composed ofthe P wave and the SV wave, and the SH type boundary acoustic wave is aboundary acoustic wave primarily composed of the SH component. Inaddition, depending on the conditions, the SH wave component and the Pwave or the SV wave component may propagate in some cases without beingcoupled with each other.

In the boundary acoustic wave, since the three displacement componentsdescribed above propagate while being coupled with each other, forexample, in a boundary acoustic wave having an acoustic velocity greaterthan that of the SH wave, the SH component and the SV component leak,and in a boundary acoustic wave having an acoustic velocity greater thanthat of the SV wave, the SV component leaks. This leaky-wave componentcauses propagation loss of the boundary acoustic wave.

Accordingly, when the acoustic velocity of the Stoneley wave isdecreased to less than the acoustic velocities of two slow transversewaves of the above two solid layers, energy of the Stoneley wave isconcentrated around electrodes disposed between the two solid layers,and a Stoneley wave having a large electromechanical coefficient k² ispropagated, such that conditions are obtained in which the propagationloss is zero. Since an electrode material having a large density has aslow acoustic velocity, when the acoustic velocity of the Stoneley waveis decreased, an electrode material having a large density is preferablyused. The present invention was made based on this understanding.

In addition, when at least one of the solid layers is made of apiezoelectric substance, and a dielectric substance including apiezoelectric substance is used as the other solid layer, the Stoneleywave is excited by the electrodes disposed between the solid layers. Theelectrodes may include comb electrodes or interdigital electrodes(interdigital transducer, IDT) as disclosed by Mikio SHIBAYAMA in“Engineering of Surface Acoustic Wave” Nov. 15, 1983, pp. 56 to 58,published by The Institute of Electronics and Communication Engineers.

FIG. 1 is a schematic front cross-sectional view of a boundary acousticwave device of one preferred embodiment according to the presentinvention, and FIG. 2 is a plan view of an electrode structure of theboundary acoustic wave device. In a boundary acoustic wave device 1, adielectric substance 3 is laminated on a plate-shaped piezoelectricsubstance 2. At a boundary between the piezoelectric substance 2 and thedielectric substance 3, electrodes defining an IDT 4 and reflectors 5and 6 are provided. The reflectors 5 and 6 are disposed on both sides ofthe IDT 4 in the propagation direction of a surface acoustic wave, andthus, in this preferred embodiment, a boundary acoustic wave resonatoris formed.

One of the unique features of the boundary acoustic wave device of thispreferred embodiment is that the IDT 4 and the reflectors 5 and 6 have alarge thickness such that the acoustic velocity of a Stoneley wave isdecreased to less than that of a slow transverse wave propagatingthrough the dielectric substance 3 and that of a slow transverse wavepropagating through the piezoelectric substance 2.

In this preferred embodiment, the thickness of the electrode isincreased so as to decrease the acoustic velocity of the Stoneley waveto less than that of each of the slow transverse waves propagatingthrough the piezoelectric substance 2 and the dielectric substance 3.Thus, the energy of the Stoneley wave is concentrated on the boundarybetween the piezoelectric substance 2 and the dielectric substance 3.Accordingly, a Stoneley wave having a large electromechanicalcoefficient k² is propagated with a low propagation loss.

In addition to the increase in thickness of the electrodes so as toenable the Stoneley wave to propagate, in the present invention, whenthe acoustic velocity of the Stoneley wave is decreased to less thanthat of each of the slow transverse waves propagating through thepiezoelectric substance 2 and the dielectric substance 3 by controllingthe duty ratio of strips defining the electrodes as described later, theStoneley wave is concentrated on the boundary between the two substancesdescribed above and is then be propagated.

Incidentally, the duty ratio of strips is a value represented by L/Pwhere L is the width of the strip and P is a distance from the center ofa space between adjacent strips to the center of a next space adjacentto the above space.

The structure shown in FIG. 1 is a simple structure in which the IDT 4and the reflectors 5 and 6 are disposed as the electrodes between thepiezoelectric substance 2 and the dielectric substance 3. With thestructure described above, boundary acoustic wave devices using aStoneley wave can be formed using a great number of materials. Forexample, in the structure disclosed in the non-patent document 2, whichis composed of SiO₂/IDT electrode/128° rotated Y plate X-propagationLiNbO₃, the Stoneley wave was not confirmed. However, although theStoneley wave may not be formed when the electrode thickness is small,the Stoneley wave may be allowed to exist when the electrode thicknessis increased. Hereinafter, with reference to particular examples, thepresent invention will be described in detail.

EXAMPLE 1

A 128° rotated Y plate X-propagation LiNbO₃ substrate, that is, havingEuler angles (0°, 38°, 0°) was prepared as the piezoelectric substance2. On this LiNbO₃ substrate, as an adhesion layer, a NiCr film wasformed by an evaporation method. Next, on this adhesion layer, a Au filmwas formed by an evaporation method, followed by patterning using alift-off method, such that the IDT 4 and the reflectors 5 and 6 wereformed. In addition, a SiO₂ film was formed by an RF magnetronsputtering method at a film-forming temperature of 200° C. so as tocover the IDT 4 and the reflectors 5 and 6.

The number of electrode finger pairs of the IDT 4 and the number ofelectrode fingers of each reflector were set to 50.5 and 51,respectively.

In addition, the crossing width of the electrode fingers of the IDT 4was set to 30 λ. On the other hand, an aperture length A (see FIG. 2) ofthe reflectors 5 and 6 was set to 30.5 λ. In this example, λ was aplacement period of the electrode fingers of the IDT 4 and thereflectors 5 and 6. In addition, the duty ratios of the IDT 4 and thereflectors 5 and 6 were each set to 0.5.

As described above, while the NiCr film, the Au film and the SiO₂ filmwere variously formed as shown in the following Table 2, one-portboundary acoustic wave devices 1 were formed.

TABLE 2

Au (λ) SiO₂ (λ) NiCr (λ) A1 0.075 4.5 0.005 A2 0.060 3.6 0.004 A3 0.0563.3 0.004 A4 0.050 3.0 0.003 A5 0.043 2.6 0.003 A6 0.038 2.3 0.003 A70.030 1.8 0.002

The impedance-frequency characteristics of each of the boundary acousticwave devices A1 to A7 shown in Table 2, which were formed as describedabove, were measured. The results are shown in FIGS. 3 to 9. Theimpedance on the vertical axis of FIGS. 3 to 9 is the value representedby20×log₁₀|Z|[dB]  (Equation 1)

In addition, in FIGS. 3 to 9, the horizontal axis represents thefrequency normalized by a resonant frequency of a response of theStoneley wave.

As shown in FIGS. 3 to 9, in the boundary acoustic wave devices A1 to A7shown in the above Table 2, a ratio Za/Zr between an impedance Za at anantiresonant point and an impedance Zr at a resonant point is in therange of about 50 dB to about 60 dB, and thus, superior resonantcharacteristics are obtained.

On the other hand, when a boundary acoustic wave device was formed in amanner similar to that for the above-described boundary acoustic wavedevices except that Al was used as the electrode material, a response ofa high order spurious mode was only confirmed, and no response of theStoneley wave could be confirmed. This result coincides with theexperimental result of the above-described non-patent document 2. Inthis example, in order to confirm the response of the Stoneley wave, adamping material was adhered to a chip surface, and the presence ofattenuation was measured.

As is apparent from the experimental results, when electrodes primarilycomposed of Au, which is heavier than Al, are used, and the thickness ofthe electrodes is set to at least about 0.03 λ, the response of theStoneley wave in the SiO₂/LiNbO₃ substrate is confirmed, which could notbe realized in the past, and superior resonant characteristics areobtained.

In addition, where a rotated Y plate X-propagation LiTaO₃ substrate or aquartz substrate was used as the piezoelectric substance 2, when thethickness of the Au film was set to at least about 0.03 λ, it wasconfirmed that the Stoneley wave can be propagated. Furthermore, withother piezoelectric substrates, when the thickness of the Au film wasset to at least about 0.03 λ, it was confirmed that the Stoneley wavecan be propagated as is the case described above.

In FIGS. 3 to 9, the normalized resonant frequency of the response ofthe Stoneley wave is about 1.0. The response at a lower frequency sidethan that of the Stoneley wave was a spurious response caused by the SHboundary acoustic wave, and a response at a higher frequency side thanthat of the Stoneley wave was a response by the high order spuriousmode.

The response of the high order spurious mode is suppressed by a method,for example, described in Japanese Patent Application No. 2003-114592.

EXAMPLE 2

In Example 1, the spurious response was generated by the SH typeboundary acoustic wave at a lower frequency side than that of theresponse by the Stoneley wave. In Example 2, this spurious response wasintended to be suppressed.

That is, in order to suppress the spurious response by the SH boundaryacoustic wave, the relationships of the Euler angle of a LiNbO₃substrate with the acoustic velocity V, the electromechanicalcoefficient k², the propagation loss α, the temperature coefficient offrequency TCF, and the power flow angle (PFA) of the Stoneley wave andthe SH type boundary acoustic wave were obtained. The calculation wasperformed based on a method disclosed in “A Method For EstimatingOptimal Cuts and Propagation Directions for Excitation and PropagationDirections for Excitation of Piezoelectric Surface Waves” (J. J.Campbell and W. R. Jones, IEEE Trans. Sonics and Ultrasonics, Vol.SU-15, No. 4 (October 1968) pp. 209 to 217). In the case of a freeboundary, the acoustic velocity and the propagation loss were obtainedbased on the assumption that the displacements, the potentials, thenormal line components of an electric flux density, and the stresses inthe up and down direction at respective boundaries between SiO₂ and Auand between Au and LiNbO₃ were continuous, the thickness of SiO₂ andthat of LiNbO₃ were infinite, and the relative dielectric constant of Auwas 1. In addition, in the case of a short-circuit boundary, thepotentials at the respective boundaries between SiO₂ and Au and betweenAu and LiNbO₃ were assumed to be zero. In addition, theelectromechanical coefficient k² was obtained by the following equation[1]. In this equation, Vf indicates the acoustic velocity of the freeboundary.k ²=2×|Vf−V|/Vf  [1]

The temperature coefficient of frequency TCF was obtained from phasevelocities V at 20° C., 25° C., and 30° C. using the following equation[2].TCF=V ⁻¹ (25° C.)×[(V(30° C.)−V(20° C.)/10° C.)]−dS  [2]

In the above equation, dS indicates the coefficient of linear thermalexpansion of the LiNbO₃ substrate in the propagation direction of theboundary acoustic wave.

In addition, the power flow angle PFA at optional Euler angles (φ, θ, ψ)was obtained from phase velocities V at angles of ψ−0.5°, ψ, and ψ+0.5°using the following equation [3].PFA=tan⁻¹ [V ⁻¹(ψ)×(V(ψ+0.5°)−V(ψ−0.5°)]  [3]

The structure used in this example was a structure in which Auelectrodes were formed on a LiNbO₃ substrate and a SiO₂ film was thenformed thereon. The thickness of the Au electrodes was set to 0.07 λ,the Euler angles were (0°, 0°, ψ), (0°, 90°, ψ), (90°, 0°, ψ), (90°,90°, ψ), (0°, θ, 0°) (0°, θ, 90°), (90°, θ, 0°(90°, θ, 90°), (φ, 0°,0°), (φ, 0°, 90°), (φ, 90°, 0°), and (φ, 90°, 90°), and φ, θ, ψ wereeach within 0° to 180°.

The results are shown in FIGS. 10 to 69.

In FIGS. 10 to 69, a value with a small letter m as a subscriptindicates a calculated value at the short-circuit boundary at which themetal film is disposed between the SiO₂ film and the LiNbO₃ substrate,and a value with a small letter f as a subscript indicates a calculatedvalue at the free boundary obtained based on the assumption that therelative dielectric constant of the metal film is 1. A value with U2 asa prefix is a calculated value of the SH boundary acoustic wave, and avalue with U3 is a calculated value of the Stoneley wave.

When the Stoneley wave is used, the SH boundary acoustic wave causes aspurious response, and ripples are generated in a pass band, or theamount of out-of-band attenuation is degraded. When theelectromechanical coefficient k² of the SH boundary acoustic wave isabout 2% or less, the degradation in properties caused by spuriousresponses of the SH boundary acoustic wave is reduced, and the boundaryacoustic wave device using a Stoneley wave can be used in a relativelywide application. In addition, when the electromechanical coefficient k²of the SH boundary acoustic wave is about 1% or less, the boundaryacoustic wave device using a Stoneley wave can be provided which can beused in a wider application. More preferably, when the electromechanicalcoefficient k² of the SH boundary acoustic wave is about 0.1% or less,since the influence of spurious responses of the SH boundary acousticwave is not substantially observed, the boundary acoustic wave deviceusing a Stoneley wave according to a preferred embodiment of the presentinvention can be used for a filter which is required to have a largeattenuation amount and for a highly precise resonator in which even aslight resonant spurious response is not allowed.

In FIGS. 10 to 69, Euler angles at which the electromechanicalcoefficient k² of the SH boundary acoustic wave is about 2% or less arepreferably in the ranges of (0°, 0°, 0°) to (0°, 0°, 180°), (0°, 90°,49°) to (0°, 90°, 131°), (90°, 0°, 0°) to (90°, 0°, 180°), (90°, 90°,48°) to (0°, 90°, 131°), (0°, −32°, 0°) to (0°, 47°, 0°), (0°, 0°, 90°)to (0°, 180°, 90°), (90°, −39°, 0°) to (90°, 39°, 0°), (90°, 0°, 90°) to(90°, 180°, 90°), (0°, 0°, 0°) to (180°, 0°, 0°), (0°, 0°, 90°) to(180°, 0°, 90°), and (0°, 90°, 90°) to (180°, 90°, 90°). Euler angles atwhich the electromechanical coefficient k² of the SH boundary acousticwave is about 1% or less are preferably in the ranges of (0°, 0°, 12.5°)to (0°, 0°, 47.5°), (0°, 0°, 62.5°) to (0°, 0°, 107.5°), (0°, 0°,132.5°) to (0°, 0°, 167.5°), (0°, 90°, 56°) to (0°, 90°, 125°), (90°,0°, −18°) to (90°, 0°, 18°), (90°, 0°, 42°) to (90°, 0°, 78°), (90°, 0°,102°) to (90°, 0°, 138°), (90°, 0°, 162°) to (90°, 0°, 180°), (90°, 90°,57°) to (90°, 90°, 127°), (0°, 13°, 0°) to (0°, 42°, 0°), (0°, 0°, 90°)to (0°, 180°, 90°), (90°, −32°, 0°) to (90°, 32°, 0°), (90°, 70°, 90°)to (90°, 110°, 90°), (12°, 0°, 0°) to (48°, 0°, 0°), (72°, 0°, 0°) to(107°, 0°, 0°), (132°, 0°, 0°) to (167°, 0°, 0°), (−18°, 0°, 90°) to(18°, 0°, 90°), (42°, 0°, 90°) to (78°, 0°, 90°), (102°, 0°, 90°) to(138°, 0°, 90°), and (0°, 90°, 90°) to (180°, 90°, 90°). Euler angles atwhich the electromechanical coefficient k² of the SH boundary acousticwave is about 0.1% or less are preferably in the ranges of (0°, 0°, 26°)to (0°, 0°, 36°), (0°, 0°, 86°) to (0°, 0°, 96°), (0°, 0°, 146°) to (0°,0°, 156°), (0°, 90°, 80°) to (0°, 90°, 111°), (90°, 90°, 110°) to (90°,0°, 119°) (0°, 26°, 0°) to (0°, 34°, 0°), (0°, 0°, 90°) to (0°, 180°,90°) (90°, −14°, 0°) to (90°, 14°, 0°), (26°, 0°, 0°) to (34°, 0°, 0°)(86°, 0°, 0°) to (94°, 0°, 0°), (146°, 0°, 0°) to (154°, 0°, 0°), (−6°,0°, 90°) to (6°, 0°, 90°), (54°, 0°, 90°) to (66°, 0°, 90°) (114°, 0°,90°) to (126°, 0°, 90°), (−7°, 90°, 90°) to (7°, 90°, 90°), (53°, 90°,90°) to (67°, 90°, 90°), and (113°, 90°, 90°) to (127°, 90°, 90°).

When LiNbO₃ substrates in the above-noted Euler angle ranges are used, aboundary acoustic wave device using a Stoneley wave can also be providedwhich has a small spurious response or which will generate no spuriousresponse.

Under all the conditions of the calculation results shown in FIGS. 10 to69, propagation losses U3-αm and U3-αf of the Stoneley wave were zero,and hence superior propagation properties are obtained.

In addition, the acoustic velocity U3-Vm of the Stoneley waveconcentrates at approximately 3,000 m/sec to approximately 3,400 m/sec,and thus, the change caused by the cut angle is small.

Accordingly, even when the cut angle is changed, an electrode thicknessH at a propagation loss of zero can be obtained by equation (4) whichwill be described later.

In addition, the temperature coefficient of frequency U3-TCFm of theStoneley wave is concentrated at approximately −30 to −40 ppm/° C., andthe change caused by the cut angle is insignificant. Accordingly, evenwhen the cut angle is changed, an electrode thickness H at which thetemperature coefficient of frequency TCF is decreased can be determinedby the equation (4).

EXAMPLE 3

A 120° rotated Y plate X-propagation LiNbO₃ substrate, that is, havingEuler angles (0°, 30°, 0°), was prepared as the piezoelectric substance2 using the calculation method described in Example 2, and taking intoaccount easy thin film formation and a function of counteracting the TCFof LiNbO₃, a SiO₂ film was selected as the dielectric substance 3. Byforming electrodes using electrode materials having various densities,boundary acoustic wave devices were formed. Subsequently, therelationships of the electrode thickness of each of the boundaryacoustic wave devices thus formed with the acoustic velocity, thepropagation loss α (dB/λ), the electromechanical coefficient k² (%), andthe temperature coefficient of frequency TCF of the Stoneley wave wereobtained. The results are shown in FIGS. 70 to 116. It is noted that thepower flow angle PFA was zero under all the conditions.

In the 120° rotated Y plate X-propagation LiNbO₃ substrate, the acousticvelocity of a longitudinal wave, that of a slow transverse wave, andthat of a fast transverse wave are approximately 6,547, 4,752, and 4,031m/sec, respectively. In addition, the acoustic velocities of alongitudinal wave and a slow transverse wave of SiO₂ are approximately5,960 and 3,757 m/sec, respectively. According to FIGS. 70 to 116, at anelectrode thickness at which the acoustic velocity of the Stoneley waveis less than 3,757 m/sec, which is the lowest acoustic velocitymentioned above, the propagation loss α of the Stoneley wave is zero.That is, by merely using an electrode material having a large density,the propagation loss α of the Stoneley wave cannot be decreased to zero,and when the electrode thickness is increased so that the velocity ofthe Stoneley wave is decreased to less than approximately 3,757 m/sec,the propagation loss α is decreased to zero.

Thus, in various preferred embodiments of the present invention, theelectrode thickness is preferably set so that the acoustic velocity ofthe Stoneley wave is decreased to less than the lowest acoustic velocityof those mentioned above, and accordingly, the propagation loss α of theStoneley wave is decreased to zero.

Furthermore, in various preferred embodiments of the present invention,by using an electrode made of a material having a large density, theacoustic velocity of a transverse wave in the electrode is decreased,and as a result, the energy of the Stoneley wave is concentrated on theelectrode. Thus, electric energy applied to the electrode and electricenergy of the Stoneley wave are efficiently coupled to each other, andas a result, a large electromechanical coefficient k² is obtained. Inaddition, since the energy is concentrated on the electrode, thereflection coefficient of the Stoneley wave reflected by electrodefingers forming the electrode is also increased. When the reflectioncoefficient of the Stoneley wave by the electrode fingers is increased,the number of electrode fingers forming a grating reflector can bedecreased. As a result, the size of the boundary acoustic wave devicecan be reduced. Furthermore, the reflection band of the reflector canalso be increased.

When the reflection of electrode fingers of the IDT 4 is not present,the frequency characteristics of the conductance of the IDT 4 arerepresented by the symmetric sinc function. On the other hand, when thereflection of the electrode fingers is present, the frequencycharacteristics of the conductance become asymmetric, and theconductance at a low frequency side of the band or at a high frequencyside thereof increases. As the reflection of the electrode fingers isincreased, the asymmetry of the above-described frequencycharacteristics is increased.

By using an IDT having internal reflection as described above, forexample, when an input side IDT and an output IDT are disposed in thepropagation direction of a boundary acoustic wave, and when reflectorsare disposed at two sides of the region in which the above-describedIDTs are arranged so as to form a longitudinally coupled filter, afilter pass band is formed which reflects the asymmetry of theconductance characteristics. In this case, as the reflection coefficientof the electrode fingers is increased, a steep band region can bedesigned. As described above, when the reflection coefficient of thefinger electrodes of the IDT is increased, steeper filtercharacteristics are easily obtained.

FIG. 117 is a graph showing the relationship between the density ρ ofthe electrode material and the electrode thickness H at which thepropagation loss of the Stoneley wave is zero. In addition, in thefollowing Table 3, the densities of various metals used as the electrodematerials are shown.

TABLE 3 Density Material (kg/m³) Al 2699 Ti 4540 Fe 7830 Ni 8845 Cu 8930Mo 10219 Ag 10500 Ta 16600 Au 19300 W 19300 Pt 21400

As shown in FIG. 117, when the thickness and the electrode material aredetermined so as to satisfy the following equation (4), the propagationloss of the Stoneley wave is decreased to zero.H[λ]>1/{1/(3×19⁷×ρ^(−2.22)+0.017)−0.4}  Equation (4)

In addition, when this type of boundary acoustic wave device ismanufactured, electrodes, such as an IDT, are formed on a piezoelectricsubstrate, such as a LiNbO₃ substrate, by a photolithographic techniqueincluding lift-off or dry etching, and on the electrodes, a dielectricfilm made of SiO₂ or other suitable material is formed by a thin-filmforming method including sputtering, evaporation, or CVD. Thus,irregularities are generated on the upper surface of the dielectric filmdue to the thickness of the IDT. In addition, the dielectric film may beobliquely grown or the film quality may become non-uniform in somecases. When the irregularities, the film growth in an oblique direction,or the non-uniformity of the film quality occurs, the properties of theboundary acoustic wave device are degraded.

In order to avoid the degradation of the properties described above, thethickness of the electrode is preferably small. According to researchperformed by the inventors of the present invention, when the thicknessH of the electrode is at least about 0.1 λ, it becomes difficult to forma dielectric thin film having superior quality. In particular, when theelectrode thickness is at least about 0.25 λ, the aspect ratio of theelectrode becomes at least 1, and it also becomes difficult to form theelectrode by using an inexpensive dry etching step or lift-off step.Furthermore, the methods and apparatuses used for dielectric thin-filmformation are limited, and as a result, it is difficult to form adielectric thin film by general RF magnetron sputtering. Therefore, theelectrode thickness is preferably about 0.25λ or less, and morepreferably about 0.1 λ or less.

As shown in FIG. 117, when an electrode material having a density ρ ofabout 4,711 kg/m or more is used, the electrode thickness H at which thepropagation loss of the Stoneley wave becomes zero is decreased to about0.25λ or less, and when an electrode material having a density ρ ofabout 7,316 kg/m or more is used, the electrode thickness H at which thepropagation loss of the Stoneley wave becomes zero is decreased to about0.10 λ or less. Hence, in the present invention, the density ρ of theelectrode material is preferably about 4,711 kg/m or more, and morepreferably about 7,316 kg/m or more.

In addition, as shown in FIGS. 72, 76, 80, 84, 88, 92, 96, 103, 107,111, and 115, at the electrode thickness H at which the condition shownby the above equation (4) is satisfied, the electromechanicalcoefficient k² is sufficiently large, such as about 3% to about 9.4%.Thus, at the electrode thickness H at which the above equation (4)holds, a boundary acoustic wave device having a sufficient band width isprovided.

In addition, as shown in FIGS. 73, 77, 81, 85, 89, 93, 97, 100, 104,108, 112 and 116, at the electrode thickness H at which the aboveequation (4) is satisfied, the absolute values of TCFs of Ag, Au, Cu,Fe, Ta, W, Ti, and Pt is about 40 ppm or less. Thus, the electrodematerial is preferably at least one material selected from the groupconsisting of Ag, Au, Cu, Fe, Ta, W, Ti, and Pt since the temperaturecoefficient of frequency characteristics is improved.

EXAMPLE 4

Next, electrodes of Au having a thickness of about 0.06 λ were formed onrespective LiNbO₃ substrates with Euler angles (φ, 30°, 0°) and Eulerangles (0°, 30°, ψ), and SiO₂ films were formed over the respectiveelectrodes. The relationships of the Euler angles θ and ψ with theacoustic velocities V, the electromechanical coefficients k², thepropagation losses α, the temperature coefficients of frequency TCF, andthe power flow angles (PFA) of the SH type boundary acoustic wave andthe Stoneley wave were measured. The results are shown in FIGS. 118 to122 and FIGS. 119 to 127. In FIGS. 118 to 127, U2 shows the results ofthe SH boundary acoustic wave, and U3 show the results of the Stoneleywave. In the entire ranges of Euler angles (0° to 90°, 30°, 0°) and (0°,30°, 0° to 90°), the propagation loss α was 0 dB/λ.

As shown in FIGS. 118 to 122, the electromechanical coefficient k² ofthe SH boundary acoustic wave is small, such as about 0.3% or less, inthe range of φ of about 0° to about 15°, and the electromechanicalcoefficient k² of the SH boundary acoustic wave is approximately 0% at φof approximately 0°. Thus, the spurious response caused by the SHboundary acoustic wave is very small. In addition, in the range of φ ofabout 0° to about 90°, TCF is superior, such as in the range of about−37 ppm/° C. to about −33 ppm/° C., and the electromechanicalcoefficient k² of the Stoneley wave is sufficiently large, such as about3.5% to about 5%. Thus, a boundary acoustic wave filter is providedwhich is preferably used as an RF filter in the narrow to the mediumbands. In addition, in the range of φ of about 0° to about 90°, thepower flow angle PFA of the Stoneley wave was small, such as about ±1.5°or less.

As shown in FIGS. 123 to 127, the electromechanical coefficient k² ofthe SH boundary acoustic wave is small, such as about 0.3% or less, inthe range of ψ of about 0° to about 14°, and the electromechanicalcoefficient k² of the SH boundary acoustic wave becomes approximately 0%at ψ of 0°. Thus, the spurious response caused by the SH boundaryacoustic wave is very small. In addition, in the range of ψ of 0° to90°, TCF is superior, such as in the range of about −36 ppm/° C. toabout −33 ppm/° C. In addition, in the range of ψ of about 0° to about45°, the electromechanical coefficient k² of the Stoneley wave issufficiently large, such as about 3.5% to about 5%, and thus, a boundaryacoustic wave filter is provided which is preferably used as an RFfilter in the narrow to the medium bands. In addition, in the range of ψof about 0° to about 90°, the power flow angle of the Stoneley wave wassmall, such as about ±1.7° or less.

In preferred embodiments of the present invention, the thicknesses ofthe dielectric substance and the piezoelectric substance are notnecessarily infinite as that of the model which was used for thecalculation and may be enough when energy of a boundary acoustic wave isconfined to near the electrodes provided at the boundary, that is, forexample, a thickness of about 1 λ or more may be enough.

In addition, according to preferred embodiments of the presentinvention, the piezoelectric substance described above may be apiezoelectric film formed on a dielectric substance.

Furthermore, in the boundary acoustic wave device according to preferredembodiments of the present invention, in order to increase the strengthor to prevent entry of corrosive gases, a protective layer may be formedoutside of the boundary acoustic wave device in the lamination directionof the dielectric substance-electrodes-piezoelectric substance laminatestructure. In this case, the boundary acoustic wave device of variouspreferred embodiments of the present invention may be sealed with apackaging material in some cases.

In addition, the protective layer described above may be formed from aninsulating material such as titanium oxide, aluminum nitride, oraluminum oxide, a metal film such as Au, Al, or W, or a resin such as aurethane, epoxy, or silicone resin.

Besides Au, Ag, Cu, and Al, the electrodes may be formed from aconductive film made of a metal, such as Fe, Ni, W, Ta, Pt, Mo, Cr, Ti,ZnO, or ITO. In addition, in order to enhance the adhesion and electricpower resistance, on an electrode layer formed from Au, Ag, Cu, Al, oran alloy thereof, a second electrode layer formed from another metalmaterial, such as Ti, Cr, or a NiCr alloy may be laminated. In thiscase, the second electrode layer may be provided between the firstelectrode layer and the piezoelectric substance, between the firstelectrode layer and dielectric substance, or at both locations mentionedabove.

Furthermore, in preferred embodiments of the present invention, theelectrode may include a sheet-shaped electrode film which defines awaveguide or a bus bar, an IDT or comb-shaped electrode exciting aboundary acoustic wave, or a reflector reflecting a boundary acousticwave.

In addition, in the specification of the present invention, as the Eulerangles (φ, θ, ψ) representing the cut surface of a substrate and thepropagation direction of a boundary acoustic wave, the right-hand Eulerangle system is used which is disclosed in “Acoustic Wave DeviceTechnology Handbook” (edited by Acoustic Wave Device Technology 150thCommittee of the Japan Society for the Promotion of Science, firstprint/first edition issued on Nov. 30, 1991, p. 549). That is, withrespect to crystal axes X, Y, and Z of LN, an Xa axis is obtained by φrotation of the X axis about the Z axis in an anticlockwise direction.Next, a Z′ axis is obtained by θ rotation of the Z axis about the Xaaxis in an anticlockwise direction. A plane including the Xa axis andhaving the Z′ axis as the normal line is set as the cut surface of asubstrate. In addition, the direction of an X′ axis obtained by ψrotation of the Xa axis about the Z′ axis in an anticlockwise directionis set as the propagation direction of a boundary acoustic wave.

In addition, as for the crystal axes X, Y, and Z of LiNbO₃ representedas the initial values of Euler angles, the Z axis is set parallel to thec-axis, the X axis is set parallel to any one of the three equivalenta-axes in three different directions, and the Y axis is set parallel tothe normal line of a plane including the X axis and the Z axis.

In addition, Euler angles equivalent to the Euler angles (φ, θ, ψ) ofLiNbO₃ of the present invention in terms of crystallography may be used.For example, according to “SAW Propagation Characteristics on LiTaO3With Arbitrary Cut”, Vol. 36, No. 3, 1980, pp. 140 to 145, since LiNbO₃is a crystal belonging to the trigonal 3 m point group, the followingequation (A) is satisfied.

$\begin{matrix}\begin{matrix}{{F\left( {\phi,\theta,\psi} \right)} = {F\left( {{{60{^\circ}} - \phi},{–\theta},\psi} \right)}} \\{= {F\left( {{{60{^\circ}} + \phi},{- \theta},{{180{^\circ}} - \psi}} \right)}} \\{= {F\left( {\phi,{{180{^\circ}} + \theta},{{180{^\circ}} - \psi}} \right)}} \\{= {F\left( {\phi,\theta,{{180{^\circ}} + \psi}} \right)}}\end{matrix} & {{Equation}\mspace{20mu}(A)}\end{matrix}$

In the above equation, F is an optional boundary acoustic-wave propertysuch as the electromechanical coefficient k², propagation loss, TCF,PFA, or a natural unidirectional property. As for PFA and naturalunidirectional property, for example, when the propagation direction isreversed, although a plus or a minus sign indicating the direction ischanged, the absolute value of the property is not changed, and thus, itis assumed that they are substantially equivalent to each other. Inaddition, although the above-described document relates to the surfaceacoustic wave, even when the boundary acoustic wave is discussed, thesymmetry of the crystal may also be dealt with in the same manner asdisclosed in the above-described document. For example, propagationproperties of a boundary acoustic wave at Euler angles (30°, θ, ψ) areequivalent to those at Euler angles (90°, 180°−θ, 180°−ψ). In addition,for example, propagation properties of a boundary acoustic wave at Eulerangles (30°, 90°, 45°) are equivalent to those at Euler angles shown inTable 4 below.

In addition, the material constant of the electrode used for calculationin preferred embodiments of the present invention is the value of apolycrystalline substance. However, even in a crystal substance such asan epitaxial film, since the crystal orientation dependence of asubstrate dominantly influences the boundary acoustic wave properties ascompared to that of the film itself, in the case of the equivalent Eulerangles represented by the equation (A), equivalent boundary acousticwave propagation properties which may not cause any practical problemscan be obtained.

TABLE 4 φ (°) θ (°) ψ (°) 30 90 225 30 270 135 30 270 315 90 90 135 9090 315 90 270 45 90 270 225 150 90 45 150 90 225 150 270 135 150 270 315210 90 135 210 90 315 210 270 45 210 270 225 270 90 45 270 90 225 270270 135 270 270 315 330 90 135 330 90 315 330 270 45 330 270 225

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A boundary acoustic wave device using a Stoneley wave, comprising: apiezoelectric substance primarily composed of LiNbO₃; a dielectricsubstance disposed on one surface of the piezoelectric substance; andelectrodes provided at a boundary between the piezoelectric substanceand the dielectric substance; wherein Euler angles (φ, θ, ψ) of thepiezoelectric substance primarily composed of LiNbO₃ are within rangesshown in the following table, and a Stoneley wave having an acousticvelocity of about 3,757 m/sec or less is used: φ (°) θ (°) ψ (°) 30 90225 30 270 135 30 270 315 90 90 135 90 90 315 90 270 45 90 270 225 15090 45 150 90 225 150 270 135 150 270 315 210 90 135 210 90 315 210 27045 210 270 225 270 90 45 270 90 225 270 270 135 270 270 315 330 90 135330 90 315 330 270 45 330 270
 225.


2. The boundary acoustic wave device according to claim 1, whereinthicknesses of the electrodes are such that the acoustic velocity of theStoneley wave is less than that of the slow transverse wave propagatingthrough the dielectric substance and that of the slow transverse wavepropagating through the piezoelectric substance.
 3. The boundaryacoustic wave device according to claim 1, wherein a duty ratio ofstrips defining the electrodes is such that the acoustic velocity of theStoneley wave is less than that of a slow transverse wave propagatingthrough the dielectric substance and that of a slow transverse wavepropagating through the piezoelectric substance.
 4. The boundaryacoustic wave device according to claim 1, wherein the electrodes areeach primarily composed of an electrode layer including at least onematerial selected from the group consisting of Ag, Au, Cu, Fe, Mo, Ni,Ta, W, Ti, and Pt.